Dilatometric Study of Reaustenitisation of High Silicon Bainitic Steels: Decomposition of Retained Austenite

Abstract

The formation of austenite during the heating of high-silicon steels with a microstructure of bainitic ferrite plates separated by carbonenriched regions of retained austenite has been studied. As the steel is heated, retained austenite decomposes into a mixture of ferrite and carbides before a temperature is reached where it becomes the more stable phase and hence can grow. The decomposition makes it necessary for the austenite to nucleate from the mixture of ferrite and carbides. In this work, dilatometric analysis, transmission electron microscopy and X-ray diffraction have helped to identify the processes that take place during the heating experiment.

Dilatometric study of phase transformations in advanced high-strength bainitic steel

“…The expansion in the dilatometric isothermal holding curve (Fig. 5) should be connected with precipitation of carbides from c phase, which is confirmed by previous findings of other authors [17,22]. During successive cooling, the volume expansion of the sample occurs from the temperature of 416°C (Fig.…”

“…They found that decomposition of austenite into the mixture of ferrite and carbides is accompanied by expansion of the specimen, visible on dilatometric curve on heating. In turn, Caballero et al [22] observed that the volume change depends on the content of C in retained austenite. They have stated that contraction of the specimen in 0.8C-2Mn-1.6Si-1.3Cr-0.3Mo steel occurs at carbon concentration above 1.5 %.…”

“…Increase of volume occurs during annealing, and therefore this cannot be a ? c transformation, which should be accompanied by a volume decrease [22,23,27]. Taking into consideration the presence of retained austenite in the initial microstructure (Fig.…”

“…Results of these investigations have often to be confirmed by detailed microscopic research because phase transformations in multiphase steels are very complex. In medium-Mn steels, the bainite is particularly difficult for unequivocal identification because it can contain carbides or may form carbide-free bainite [8,[18][19][20][21][22]. Films of retained austenite instead of carbides occur between laths of bainitic ferrite.…”

Phase Equilibrium and Austenite Decomposition in Advanced High-Strength Medium-Mn Bainitic Steels

“…However, further work is intended to explain in detail the presence of different phases, the effect of strain, composition, and cooling rates. It should be taken into account that plastic deformation shifts diffusion-controlled transformations to a shorter time [31,32]. JMatPro software does not enable the user to take into take into account a grain size after plastic deformation, which is usually successfully decreased during multistep deformation tests [21,24].…”